Page:Popular Science Monthly Volume 10.djvu/681

Rh You see, therefore, that we give off more heat by conduction in the open air than in a room, and in the latter proportionately more by radiation and evaporation.

The power of conduction is best appreciated when we change the air for some other fluid medium, which is a better conductor than air, and more capable of absorbing heat, I mean water. In air of a few degrees of heat, we can feel pretty comfortable with moderately warm clothes; but, if with the same amount of clothing we were to get into water of the same temperature, we should feel painfully cold, and should probably be frozen to death in a few hours, although our loss by evaporation would have ceased entirely, and that by radiation nearly so. In hot climates, therefore, a daily bath is of great service for the necessary cooling of our body, even if the water is not cooler than the atmosphere.

In the air also the loss of heat by conduction is the greater the lower the temperature, and the greater the velocity of the air which flows around us. This explains on the one side why it appears superfluous in a calm and cool air to make use of a fan, while this expedient acts so beneficially at higher temperatures; and on the other why, as a rule, a warm air in motion appears much cooler than a calm one of equal temperature. Think of the sultriness before a thunder-storm, as long as the air is at rest, and how differently we feel as soon as the first wind rises. The air is not yet cooler, not less saturated with vapor than before, and still it deprives us of so much more heat that we deem it less sultry, even cool, only because it travels over us faster.

When we fan ourselves in a hot and damp air, the same thing takes place—then, also, a greater amount of air passes over us in a given time than if we leave the air to its own motions. The fan changes nothing in the temperature and moistness of the air, it only increases its velocity, and in consequence the abstraction of heat, and thus affords us coolness chiefly on the uncovered or only slightly covered parts of our bodies; therefore, ladies have more reason for using it than the stronger sex.

As long as the air is our surrounding medium, an increased evaporation associates itself with the increased loss by conduction, at least as long as the circulation of the blood in the skin remains active and the air is not saturated with moisture. The fan scarcely ever cools by increased conduction alone, but also by increased evaporation. Therefore, fanning with dry air is much more cooling than fanning with a moist air of equal temperature. We all know how much quicker wet roads and wet clothes dry when there is a good wind. However rapid the motion of moist air may be, it does not dry. When our body is bathed in perspiration the fuller condition of the skin occasions an increased transfer of heat from the dilated blood-vessels to the surrounding air by conduction, but generally also by evaporation.

In southern climes, at the hottest and moist time of the year, when